1. Field of the Invention
This invention relates in general to a method and apparatus for performing self servo rewrites, and more particularly to a method and apparatus for detecting servo errors and rewriting the servo data.
2. Description of Related Art
Disk drives typically employ electromagnetic read/write heads to store and recover information from the recording surfaces of rotating disks. The disks generally include a high coercivity magnetic storage layer at each planar surface. Data is normally stored along concentric data tracks in the magnetic storage layer. For disk drives having more than one data recording surface, the set of tracks at a common radial location is referred to as a cylinder. Head positioning relative to a track or cylinder is usually performed using a closed-loop servo scheme, so that the head(s) can be precisely positioned for track following. Servo data may be recorded on a dedicated disk surface (i.e., "dedicated-servo") or embedded in every track of every recording surface (i.e., "embedded-servo"). The latter servo scheme tends to provide more accurate head positioning and in the case of a disk drive having a few disks, an embedded-servo scheme will tend to use a smaller percentage of the potential data storage area for servo overhead. Accordingly, disk drives incorporating embedded-servo schemes currently dominate the market.
Typically a data track is divided up into a number of blocks for storing user data. The actual number of data blocks depends upon the size of each block, as measured in the number bytes stored therein, the radius of the track, the data transfer rate, etc. The embedded servo sectors may follow the data rates of the data blocks, or they may be recorded e.g. at a single rate across the radial extent of the disk, thereby interrupting and splitting fixed length data blocks into segments.
In order for an embedded servo drive to operate properly, the servo information defining the position of the data tracks must be written with great accuracy. Typically, the servo information is written on each surface which is transverse to and interspersed between data tracks. This information typically includes a sector identifier (SID), binary position information and an analog position error sensing (PES) burst pattern. Other formats and servo patterns are also employed within embedded servo disk drives. Coarse positioning is provided by the binary position information with fine head positioning relative to a track center can be determined and corrected, if necessary, by reading and noting the respective amplitudes and timings of these latter offset bursts.
Traditionally, the machine used to write the embedded servo information is called servowriter. A servowriter typically includes a large, massive granite base to minimize the effects of vibration, precision fixtures to hold the target disk drive, a precision laser interferometer based actuator arm positioning mechanism to position the arms radially with respect to the axis of rotation of the disks in the drive, and an external clock head to position the servo information on time(i.e., generally in a radial direction). Once the set is written, a read back procedure is employed while the disk drive head and disk assembly remains at the servo writer station in order to verify that the servo information have validly been written and will be useful by the disk drive head positioner servo loop during subsequent drive operations. Due to the incorporation of large granite structures, laser based sensors, and high precision fixtures, present servowriters are extremely large and expensive.
During the manufacture of the disk drive, the above described servo information is permanently written on the disk by the servowriters. Unfortunately, sometimes the information written is erroneous. In this case it is usually only a few sectors on the drive that are miswritten; but this can be enough that the drive needs to be reservowritten. Worse, the false servo information may lead to rework in some other, expensive, way.
One proposed solution is to exhaustively check each file at the servo writer itself. A disadvantage of this solution is that the extensive checking is time consuming, which ties up capital equipment and so adversely effects profit margins for the disk drives. In effect, this type of check will more than double the cost to servowrite the file. Furthermore, even if the drive passes the manufacture test, asperities at the servo region may develop on the disk at a later time.
It can be seen that there is a need for a method and apparatus that corrects errors that occur during servowriting without requiring the disk to be re-servowritten, i.e., rewriting the servo information for the entire disk.
It can also be seen that there is a need for a method and apparatus to detect defective servo sectors need to be rewritten and to correct defective servo sectors by writing in the servo fields.